Well, I guess there's nothing wrong with registering it strictly to deprecate it, since folks who use it are not likely to be those who pay attention to registrations, anyway ;)
But text/xsl, as we all know[1] runs afoul of the convention that text/... formats be human readable. Then again with text/html, the beast has surely escaped the barn.
[2] a sample of such consideration is in RFC 3023: "
NOTE: Users are in general not used to text containing tags such
as <price>, and often find such tags quite disorienting or
annoying. If one is not sure, the conservative principle would
suggest using application/* instead of text/* so as not to put
information in front of users that they will quite likely not
understand."
